The present invention relates to a process for polymerizing olefins such as ethylene and propylene, and more particularly to a process for polymerizing olefins using a multi-stage polymerization apparatus including a vapor-phase polymerization reactor in which the composition of gas in a second- or subsequent-stage vapor-phase polymerization reactor is easily controlled.
Of the conventional industrial processes for polymerizing olefins represented by ethylene and propylene, a vapor-phase polymerization process has been particularly noticed because the recent improvement in the activity of the catalyst for this process renders the deashing of polymer product for removing catalyst residue unnecessary. However, in the conventional continuous multistage vapor-phase polymerization processes, the composition of gas in an earlier-stage polymerization reactor directly influences that of a later-stage polymerization reactor. The control of the composition of gas in the later-stage polymerization reactor sometimes becomes labor-intensive and costly.
For example, in ordinary production of polymers having a broad molecular weight distribution, a low-molecular weight polymer produced in an earlier-stage reactor is further polymerized into a high-molecular weight polymer in a later-stage reactor. The earlier-stage polymerization is performed in the presence of hydrogen as a molecular weight modifier in a high concentration. On the other hand, the later-stage polymerization is required to be performed in a low hydrogen concentration. Therefore, the accompanying hydrogen during the transfer of the polymers obtained in the earlier-stage reactor to the later-stage reactor should be reduced to a large extent.
In the production of copolymers such as ethylene-propylene copolymers, it is also required to control the composition of gas in each stage polymerization reactor.
Under these circumstances, various methods have been proposed to control the composition of gas in the later-stage vapor-phase polymerization reactor. For example, Japanese Patent Application Laid-Open No. 59-230010 discloses a method of reducing the concentration of hydrogen (molecular weight modifier) in the accompanying gas from the earlier-stage reactor by providing an intermediate receiver (purge vessel) between the earlier-stage and later-stage polymerization reactors, thereby removing the accompanying gas and transferring only the polymer to the later-stage reactor. U.S. Pat. No. 4,420,592 discloses a method in which the accompanying gas from the earlier-stage reactor is diluted with an inert gas in the intermediate receiver to reduce the hydrogen concentration, and then transferred to the later-stage reactor together with the polymer. Japanese Patent Application Laid-Open No. 7-118342 discloses a method using an intermediate receiver comprising a gas vessel, a powder vessel and a valve disposed between the vessels. The accompanying gas is removed by the open-close operation of the valve, and the polymer in the powder vessel is forced into the later-stage reactor by a pressure gas circulated from the later-stage reactor. However, these conventional methods are still unsatisfactory since they require the intermediate reactor, the complicated valve operation, etc.
An object of the present invention is to provide a process for polymerizing olefins using a multistage polymerization apparatus including a later-stage vapor-phase polymerization reactor, in which the composition of gas in the later-stage vapor-phase polymerization reactor is easily controlled to ensure stable production of polymers having a desired composition.
The inventors have already proposed, in Japanese Patent Application Laid-Open No. 11-209414, a method for controlling the composition of gas in a later-state vapor-phase polymerization reactor in a continuous multistage polymerization of olefins. In the proposed method, after liquefying a part (heavy component) of a multicomponent gas taken out of the later-stage vapor-phase polymerization reactor by compressing and/or cooling, at least a part of the non-liquefied gas is removed from the reaction system and the remaining gas is, together with the liquefied gas, returned to the later-stage polymerization reactor, thereby controlling the composition of gas in the later-stage vapor-phase polymerization reactor. With this method, it has become possible to simultaneously carry out the control of the composition of gas in the later-stage polymerization reactor and the temperature control of the later-stage polymerization reactor (removal of polymerization heat).
The inventors have continued the study on this method, and found that the composition of gas in the later-stage vapor-phase polymerization reactor is effectively controlled by mixing a multicomponent gas taken out of the later-stage vapor-phase polymerization reactor with an inert gas such as nitrogen which is lighter than at least one component (e.g., olefins) of the multicomponent gas. The present invention has been accomplished based on this finding.
Thus, in a first aspect of the present invention, there is provided a process for polymerizing olefins using a plurality of series-connected polymerization reactors in which at least one of second and subsequent reactors is a vapor-phase polymerization reactor containing a multicomponent gas, the process comprising:
taking out the multicomponent gas from the vapor-phase polymerization reactor;
mixing the multicomponent gas with an inert gas which is lighter than at least one component of the multicomponent gas, thereby obtaining a mixed gas;
compressing and/or cooling the mixed gas to liquefy a part of the multicomponent gas;
discharging at least a part of a gaseous mixture comprising the inert gas and an unliquefied multicomponent gas out of a reaction system; and
returning a fluid comprising the remainder of the gaseous mixture and the liquefied multicomponent gas to the vapor-phase polymerization reactor.
In a second aspect of the present invention, there is provided a process for polymerizing olefins using a plurality of series-connected polymerization reactors in which at least one of second and subsequent reactors is a vapor-phase polymerization reactor containing a multicomponent gas comprising an olefin and hydrogen, the process comprising:
taking out the multicomponent gas from the vapor-phase polymerization reactor;
mixing the multicomponent gas with an inert gas which is lighter than the olefin in the multicomponent gas, thereby obtaining a mixed gas;
compressing and/or cooling the mixed gas to liquefy at least a part of the olefin in the multicomponent gas;
discharging at least a part of a gaseous mixture comprising the inert gas and the unliquefied multicomponent gas out of a reaction system; and
returning a fluid comprising the remainder of the gaseous mixture and the liquefied olefin to the vapor-phase polymerization reactor.